1. Field of the Invention
The present invention relates to a memory element including a semiconductor device and a signal processing circuit including the memory element.
2. Description of the Related Art
It has recently been discovered that a transistor formed using an oxide semiconductor with a large band gap has significantly high off-state resistance, and it has been suggested that the transistor is used to form a memory element or a signal processing circuit (see References 1 to 4). In addition, with a highly thin polycrystalline silicon film, a transistor whose off-state resistance is three or more orders of magnitude higher than that of a conventional transistor has been formed, and its application to a gain cell has been suggested (see Reference 5).
Owing to the high off-state resistance of the transistor in such a memory element, it takes a long time for electric charge accumulated in a capacitor to be lost, and this makes it possible to eliminate the consumption of electric current required for a conventional flip-flop circuit such as an SRAM to maintain memory storage and makes it possible to reduce power consumption. Alternatively, a very large capacitor required for a DRAM is not necessary, which allows the size of a circuit to be reduced, the manufacturing process to be simplified, and the yield to be improved.